The present invention relates to a cleaning device installed in an electrophotographic copier for removing an untransferred part of toner which remains on a photoconductive element. More particularly, the present invention is concerned with a residual toner collecting device capable of effectively self-cleaning electrode members which are apt to be contaminated by a part of toner removed from the surface of the photoconductive element and various impurities brought by the cleaning device, thereby maintaining stable collection performance.
Generally, in an electrophotographic copier of the type developing an electrostatic latent image formed on a photoconductive element by means of toner to produce a visible toner image, untransferred toner, if small in amount, remains on the surface of the photoconductive element even after the transfer of the toner image to a paper sheet. For the removal of the untransferred or residual toner, a copier of type described is usually furnished with a cleaning device which may comprise a magnetic brush or a fur brush, for example. Such a cleaning device, in turn, is furnished with a toner collecting device adapted to collect the removed residual toner so as to allow it for reuse. The toner collector, which has been proposed in various configurations, may comprise a scavenger roller, a blade, a flat counterelectrode, and an air filter. The scavenger roller is applied with a dc voltage opposite in polarity to a charge deposited on toner, so that the residual toner removed from the photoconductive element by, for example, a fur brush may be attracted onto the scavenger roller. The toner deposited on the scavenger roller is scraped off by the blade. The counterelectrode is located to face the scavenger roller and applied with a dc voltage common in polarity to the charge on the toner. Dust and other impurities entrained by a stream of air generated by the rotating fur brush are effectively separated from the toner by the air filter.
In the particular toner collector described above, the impurities entrained by the air stream mentioned include small pieces, or naps, of the fur brush which are produced during rotation of the brush. Since the fur brush is usually made of Teflon which has a tendency to charging to negative (-) polarity, naps of the brush stick to the flat counterelectrode which is usually applied with a positive (+) dc voltage, tending to accumulate thereon with the lapse of time. Meanwhile, the naps of the Teflon brush are as long as about 30-400 microns which is far larger than the diameter of toner particles which is about 6-10 microns and, additionally, the charge level of the toner particles is substantial. This allows the toner removed from the photoconductive element by the fur brush to easily stick to naps separated from the brush.
For the above reasons, the naps of the Teflon brush carrying toner particles therewith stick to the counterelectrode before sucked by the air filter. Accumulating gradually on the counterelectrode, the naps of the brush reduces the clearance between the counterelectrode and the scavenger roller little by little. For example, an initially designed clearance of about 8 millimeters may even be reduced to about 1-3 millimeters. The reduced clearance serves to increase the velocity of the air stream developed by the fur brush so that the toner removed by the fur brush from the photoconductive element is entrained by the rapid air stream to be entirely sucked by the air filter without being caught by the scavenger roller.
In the above situation, the toner removed from the fur brush fails to be collected for reuse, while the toner flown past the scavenger roller stays on the filter to significantly deteriorate its performance and make the service life far shorter than expected.
In an electrophotographic copier which uses, as a developer, toner particles having diameters not larger than about 2 microns, for example, the residual toner tends to remain deposited on the scavenger roller due to its intense adhesion and because the blade cannot easily scrape it off. As a result, capture of the toner by the scavenger roller becomes difficult thereafter, limiting the efficiency of toner collection.
As discussed above, performance of a prior art residual toner collector used in combination with a cleaner of the type using a fur brush or a magnetic brush is quite limited by the fact that a part of toner and various impurities floating in the air tend to stick to the surface of the scavenger roller and/or the counterelectrode. No implementations have been proposed to settle this problematic situation. Stated another way, no propositions have been made concerning a method or an apparatus for effectively removing a part of toner and impurities which deposite on the scavenger roller or the counterelectrode.